Conventional nuclear rocket engines employ a nuclear fission reactor to heat the rocket propellant, typically hydrogen, to extremely large temperatures. The hot hydrogen is then expelled from a nozzle into space at supersonic speed to create thrust for the rocket. To conserve nuclear fuel and propellant, space mission operations will typically only require short duration engine firings. The reactor is turned on for a brief period to generate thrust to propel the rocket to a cruise velocity in space and then the reactor is shut down.
Shutting down nuclear rocket engines during the space mission has presented many design challenges. One challenge results from the fact that nuclear reactors cannot be immediately turned off. Delay neutrons and daughter products of the fission reaction generate power long after the reactor ceases to operate. This energy or heat must be removed from the rocket to prevent overheating and destruction of the engine. In addition, the engine feed system (pumps and turbines) must be shut down as the reactor power decays to throttle propellant flow and to prevent the pumps from surging. Shutting down the feed system, however, makes it makes it extremely difficult to remove the reactor heat from the engine.
Another challenge created by shutting down rocket engines in flight is cooling the nuclear reactor. Byproducts of the nuclear reaction (waste heat) continuously heat the components of the reactor during engine firing and long after the engine has been shut down. To solve this problem, liquid or gaseous hydrogen (apart from the actual propellant) is typically used to cool the reactor. The hydrogen is directed through the reactor, which transfers some of its heat to the hydrogen, and is then expelled from the rocket into space. This process continues until the reactor temperature has been brought down to a safe level. One problem with this method is that cooling the reactor can take a long time (from a few hours to a few days). Thus, an enormous amount of hydrogen must be stored in the rocket to cool the reactor. This large volume of hydrogen increases the weight of the rocket which decreases mission performance (payload/initial mass) and increases mission cost.
To decrease the amount of hydrogen needed to cool the reactor, existing systems have attempted to alternate between undercooling and overcooling the reactor. In this scheme, the reactor is allowed to heat up until it reaches a very high temperature and is then quickly cooled down with extremely cold propellant. Hydrogen is conserved because it is not continuously pumped through the reactor and into space. Alternating between undercooling and overcooling the reactor, however, can create thermal shocks that damage reactor components and create flow instabilities, thereby decreasing the life of the nuclear engine.